mpg accuracy?

[mikejay]

How accurate is the reading in an Octavia? Went to Carlisle Monday, with a heavy foot mainly motorway returned 52mpg, coming back today barely saw 40! That's a remapped cr. My old remapped fabia would always tend to return 45 to 50 mpg no matter how heavy footed I was. Why is this?

[booke23]

Certainly on the PD engines the trip computer accuracy is all over the place. Mine was over reading by about 8%, but I have corrected it with VCDS. I have heard of others that under read by a similar amount. No rhyme or reason it it, just varies from car to car.

 

However the CR engined cars seem to all have fairly accurate trip computers. But your remap may be affecting trip computer accuracy. The only way to know is to do brim to brim calculations for 5 tankfulls or more to get an accurate picture.  

[littleade]

+1 on the above. Brim to brim is far more accurate tho my maxidot is showing a tad over 49mpg on the long term setting which I don't reset which is comparible with what I'm calculating manually each time I fill up so as brook says your nice remap is probably getting your maxidot in a twist. Be interesting to see what you find out when you do the comparison actual vs maxidot.

[philbes]

After 5,300 miles in 4½ months the Maxidot in my car is showing an overall consumption of 47.2mpg but the true calculated figure is 43.78 .  So readout is almost  8% optimistic.  I don't drive it for economy but would hope for a true 45+.

[flying pig]

I use an app called Road Trip Lite. Upon every refuel I input the mileage, how many litres uploaded & price per litre. The app does the rest. Here's an example for my 1.9 PD (105 bhp). My journey's are mostly on motorways / dual carriageways travelling at no more than 70 mph.

Edited 14 November, 2013 by flying pig

[mikejay]

I'll download that

[Lew0-VRS]

Mine since I bought it cr 170 mapped mixed driving. And both me and swmbo don't hang about.

[rogerzilla]

It's quite accurate on the 1.4TSI, to the extent that I don't bother with the brim-brim anymore.

 

On my old BMW it was a consistent 15% optimistic - they are notoriously bad on BMWs.  On the MX-5 it is pretty good and varies between 0-2% pessimistic, i.e. brim-brim always gives a slightly higher mpg than the computer (it's also always exceeded its official combined mpg from new).

[Metblackrat]

The odometer is inaccurate. Petrol pumps are inaccurate. Any calculations based on the readings from these devices will also be inaccurate.

[Lew0-VRS]

The odometer is inaccurate. Petrol pumps are inaccurate. Any calculations based on the readings from these devices will also be inaccurate.

Some Replies on the internet are also inaccurate.

[booke23]

The odometer is inaccurate. Petrol pumps are inaccurate. Any calculations based on the readings from these devices will also be inaccurate.

 

Odometers are generally very accurate. You can test it on a 10 mile motorway run.....count the mile markers.

 

Trading standards regularly test fuel pumps for accuracy, and report that 99% of pumps are dispensing exactly the right amount of fuel.....those that don't mostly over dispense.  

[mikejay]

Ok then so we have come to the conclusion that there roughly right, so why the big difference in economy comin back then going?

[GriffoVRS]

Long term comparison from filling up shows Maxidot indicating about 5% over. On my PD vRS is was virtually bob on.

 

Most back to back runs tend to show comparable economy, but every now and again a run will have a lower mpg. The biggest factor seems to be the weather, especially if it is raining (which I put down to the extra drag on the tyres, etc,).

[Web Ferret]

Mine reads 10% over compared to the fuely app as my sig line.

 

I suggest it was the wind. Behind you on the way up, against you on the way down. Rain as above can make a difference. 

[Metblackrat]

Odometers are generally very accurate. You can test it on a 10 mile motorway run.....count the mile markers.

Trading standards regularly test fuel pumps for accuracy, and report that 99% of pumps are dispensing exactly the right amount of fuel.....those that don't mostly over dispense.

I regularly test/calibrate speedometers/odometers. They are never very accurate, some over read by 10% at 70mph.

Petrol pumps are allowed to over/under dispense by 0.3%. If you factor these errors in you get bigger errors out.

Edited 14 November, 2013 by Metblackrat

[booke23]

Ok then so we have come to the conclusion that there roughly right, so why the big difference in economy comin back then going?

 

As Griffo says, discrepancies between an outward and return MPG on the same route are generally down to weather. Wet weather makes a noticeable difference, and wind makes a big difference too. A 15 mph tail wind on the way there becomes a 15 mph headwind on the way back, which has a noticeable affect on economy.   

 

 

I regularly test/calibrate speedometers/odometers. They are never very accurate, some over read by 10% at 70mph.

 

The speedometers and odometers on VAG cars are independent of each other. So if the Speedo over reads by 10% (which is very common) the odometer almost always does not.....in fact it is usually highly accurate. 

Edited 14 November, 2013 by booke23

[Ultrasonic]

As well as wind and rain, another factor that could give different there and back mpg figures is air temperature. Colder air is more dense leading to higher aerodynamic drag - the higher the speed the more significant this will be.

 

That said 52 mpg to 40 mpg is a BIG drop.

 

Did you refuel in Carlisle for the return leg?

 

(Oh, and the Maxidot on my PD Octavia is about 20% optimistic. Most are better than this but it shows how wildly innaccurate they can be. It does appear that more modern ones are more accurate though.)

Edited 14 November, 2013 by Ultrasonic